Modern semiconductor components such as microprocessors, memories and so forth are formed with millions of transistors and other components. To enable operation of a transistor, a voltage is applied to a gate of the transistor. Generally, this voltage must be greater than a so-called threshold voltage in order to turn the transistor on. If instead the gate voltage is less than the transistor's threshold voltage, generally the transistor will remain off.
Variations in the threshold voltage of metal oxide semiconductor field effect transistors (MOSFETs) can cause significant variation in circuit performance and leakage power. No known methods currently exist for monitoring variations in threshold voltage across a product. A discrete device placed in a test structure in a wafer scribe line of a wafer that includes many semiconductor die can provide a very coarse estimation of threshold voltage variation. However, measuring discrete devices on die creates a technical challenge, and may also require a long testing time, and is thus not feasible for high volume manufacturing (HVM) testing.